X-ray Opaque Barium-Free Glasses and Uses Thereof

ABSTRACT

Zirconium-containing BaO- and PbO-free X-ray opaque glasses having a refractive index n d  of about 1.518 to about 1.533 and a high X-ray opacity with an aluminum equivalent thickness of at least about 180% are provided. Such glasses are based on a SiO 2 —Al 2 O 3 —B 2 O 3  system with additions of K 2 O, ZrO 2 , and one or both of La 2 O 3  and Cs 2 O. Such glasses may be used, in particular, as dental glasses or as optical glasses.

RELATED APPLICATION

This application claims priority to and benefit of German Application No. 10 2009 008 951.9 filed on Feb. 13, 2009, the contents of which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to barium- and lead-free X-ray opaque glasses and to uses thereof.

BACKGROUND OF THE INVENTION

Plastic dental compositions are increasingly being used for dental restoration in the dental sector. Such plastic dental compositions usually include a matrix of organic resins and various inorganic fillers. Inorganic fillers predominantly comprise powders of glasses, (glass-) ceramics, quartz or other crystalline substances (e.g. YbF₃), sol-gel materials or aerosils, which are added to the plastic composition as filling material.

The use of plastic dental compositions is intended to avoid possible harmful side-effects of amalgam and to achieve an improved aesthetic impression. Depending on the plastic dental compositions selected, they can be used for different dental restoration purposes, for example, for tooth fillings, as well as for securing parts, such as crowns, bridges and inlays, onlays etc.

The filling material per se is intended to minimize the shrinkage caused by polymerization of the resin matrix during curing. For example, if there is a strong adhesion between the tooth wall and filling, excessive polymerization shrinkage can lead to the tooth wall breaking. If the adhesion is inadequate, excessive polymerization shrinkage may result in the formation of peripheral gaps between the tooth wall and filling, which can promote secondary caries. Furthermore, certain physical and chemical demands are imposed on the fillers.

It is desirable to process the filling material to form powders that are as fine as possible. The finer the powder, the more homogenous the appearance of the filling. At the same time, the polishing properties of the filling are improved, which in addition to reducing the surface area available for attack also leads to improved resistance to abrasion and therefore to a longer-lasting filling. To enable the powders to be processed successfully, it is also desirable for the powders not to agglomerate. This undesirable effect tends to occur with filling materials produced using sol-gel processes.

Furthermore, it is advantageous if filler particles are coated or at least partially coated with functionalized silane, since this facilitates formulation of dental compositions and improves the mechanical properties.

Furthermore, the refractive index and color of the entire plastic dental composition, including fillers, should be as well matched as possible to the natural tooth material, so that it is as indistinguishable as possible from the surrounding, healthy tooth material. The grain size of the pulverized filler being as small as possible also helps to achieve this aesthetic criterion.

It is also important for the thermal expansion of the plastic dental composition in the typical range of use, i.e. usually between −30° C. and +70° C., to be matched to that of the natural tooth material in order to ensure that dental restoration measures are sufficiently able to withstand temperature changes. Excessively high stresses caused by temperature changes also can cause formation of gaps between plastic dental compositions and the surrounding tooth material, which in turn can form sites of attack for secondary caries. In general, fillers with the lowest possible coefficient of thermal expansion are used to compensate for the high thermal expansion of the resin matrix.

Good chemical resistance of the fillers with respect to acids, alkalis and water and good mechanical stability under load, such as, for example, during movement produced by chewing, can also contribute to a long service life for dental restoration measures.

Furthermore, for the treatment of patients, it is imperative that dental restoration measures can be seen in an X-ray image. Since the resin matrix itself is generally invisible in an X-ray image, the fillers must provide the required X-ray absorption. A filler of this type which provides sufficient absorption of X-radiation is described as X-ray opaque. Constituents of fillers, for example, certain components of a glass, or other substances, are generally responsible for X-ray opacity. Such substances are often referred to as X-ray opacifiers. A standard X-ray opacifier is YbF₃, which can be added to the filler in crystalline, milled form.

According to International Standard DIN ISO 4049, the X-ray opacity of dental glasses or materials is quoted in relation to the X-ray absorption of aluminum as aluminum equivalent thickness (ALET). The ALET is the thickness of an aluminum sample which has the same absorption as a 2 mm-thick sample of the material to be tested. An ALET of 200% therefore means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 4 mm. Analogously, an ALET of 150% means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 3 mm.

Because plastic dental compositions in use are usually introduced into cavities from cartridges and then modeled in the cavities, such compositions should be at least somewhat thixotropic in the uncured state. This means that viscosity decreases when pressure is exerted, while it is dimensionally stable without the action of pressure.

Among plastic dental compositions, a distinction also should be drawn between dental cements and composites. In the case of dental cements, also known as glass ionomer cements, the chemical reaction of fillers with the resin matrix leads to curing of the dental composition, and consequently the curing properties of the dental composition. Thus, their workability is influenced by the reactivity of the fillers. This often involves a setting process which is preceded by a radical surface curing, for example, under the action of UV light. Composites, also referred to as filling composites, contain by contrast fillers which are as chemically inert as possible, since their curing properties are determined by constituents of the resin matrix itself and a chemical reaction of the fillers often disrupts this.

Because glasses, due to their different compositions, represent a class of materials with a wide range of properties, they are often used as fillers for plastic dental compositions. Other applications as dental material, either in pure form or as a component of a material mixture, are also possible, for example, for inlays, onlays, facing material for crowns and bridges, material for artificial teeth or other material for prosthetic, preservative and/or preventive dental treatment. Glasses of this type used as dental material are generally referred to as dental glasses.

In addition to the dental glass properties described above, it is also desirable for this glass to be free from barium and/or barium oxide (BaO), which are classified as harmful to health, and also from lead and/or lead oxide (PbO) and from other barium and lead compounds.

In addition, it is also desirable for a component of dental glasses to be zirconium oxide (ZrO₂). ZrO₂ is a widely-used material in the technical fields of dentistry and optics. ZrO₂ is readily biocompatible and is distinguished by its insensitivity to temperature fluctuations. It is used in a wide variety of dental supplies in the form of crowns, bridges, inlays, attachment work and implants.

Dental glasses therefore represent glasses of particularly high quality. Glasses of this type also can be used in optical applications, particularly if such applications benefit from the X-ray opacity of the glass. Since X-ray opacity means that the glass absorbs electromagnetic radiation in the region of the X-ray spectrum, corresponding glasses simultaneously act as filters for X-radiation. Sensitive electronic components can be damaged by X-radiation. In the case of electronic image sensors, for example, the passage of an X-ray quantum may damage the corresponding region of the sensor or result in an undesirable sensor signal which can be perceived, for example, as an image disturbance and/or disturbing pixels. For specific applications it is therefore necessary, or at least advantageous, to protect electronic components against X-radiation by using corresponding glasses to filter said components out from the spectrum of the incident radiation.

A number of dental glasses and optical glasses are known from the prior art.

For example, WO2005/060921 A1 describes a glass filler which, in particular, is intended to be suitable for dental composites. However, this glass filler must contain only 0.05 to 4 mol % alkali metal oxides. This low alkali metal oxide content of the metal oxides, in particular in combination with ZrO₂, makes the dental glass more inclined to segregate. The segregated regions act as centers for scattering light that passes through, analogous to the Tyndall effect. This may create unfavorable consequences for the optical properties of the dental glass and the aesthetics of the plastic dental compositions produced with segregated dental glasses therefore cannot satisfy relatively high demands.

EP 0885606 B1 describes an alkali metal silicate glass which serves as filling material for dental material. The limited B₂O₃ content of 0.2 to 10% by weight described in this reference makes it difficult to melt glass with a high SiO₂ content, making it expensive and uneconomical to produce such glass.

U.S. Pat. No. 5,976,999 and U.S. Pat. No. 5,827,790 relate to glass-like ceramic compositions used, inter alia, for dental porcelain. These references state that CaO and LiO₂ must be present in amounts of at least 0.5% by weight and 0.1% by weight, respectively. In addition to the two main additional components from the group consisting of ZrO₂, SnO₂ and TiO₂, a content of CaO therein of at least 0.5% by weight appears to be essential. These components bring about X-ray opacity and an increased refractive index n_(d). Even small amounts of CaO enhance the mechanical properties, such as for example the Vickers hardness. However, an increased Vickers hardness is disadvantageous during the milling process since the milling bodies are subjected to increased abrasion and the duration of the process increases.

Certain chemically inert dental glasses for use as filler in composites are described in DE 198 49 388 A1. The glasses proposed therein must contain significant proportions of ZnO and F which can lead to reactions with the resin matrix, which can in turn have effects on their polymerization properties. In addition, the SiO₂ content is limited to 20-45% by weight so that such glasses can contain sufficient X-ray opacifier and F.

DE 4443173 A1 describes glass which has a high ZrO₂ content of more than 12% by weight and which contains other oxides. Fillers such as these are too reactive, in particular for most modern epoxy-based dental compositions in which excessively rapid, uncontrolled curing may occur. Zirconium oxide in this amount tends to become devitrified. This brings about phase segregation, possibly with nucleation and subsequent crystallization.

WO 2005/080283 A1 describes glass with a refractive index gradient for optical elements. However, the claimed glass contains 12-50% by weight B₂O₃ which impairs the chemical resistance of the glass and is therefore unsuitable for dental glasses.

US 2003/0161048 A1 describes a glass (lens) with a refractive index gradient. This is achieved by the diffusion of silver. Mandatory for this purpose are at least 3 mol % of the easily exchangeable Li₂O and a total Li₂O and Na₂O content of 3 to 65 mol %. Because it is easy to exchange Li₂O, it is desirable in dental glasses and also other weathering-resistant glasses to dispense with the Li₂O component. Li₂O is quickly leached from the glass and, in the presence of tooth material, may reduce the durability thereof. Furthermore, the glass itself is destabilized by such leaching and the transparency may also be adversely affected. Thus, leaching should be avoided in optical glasses.

DE 3501898 C2 describes glass for optical waveguides, but this glass must contain F. For the reasons already mentioned, F is undesirable in dental glasses.

JP 2006-052125 A2 relates to a silicate substrate glass for flat panel displays which, for adjusting viscosity, contain considerable proportions of alkaline-earth metal oxides, i.e. the sum total of MgO, CaO, SrO and BaO is 15 to 27% by weight. The viscosity curve of this glass is very steep, which means that there is only a narrow temperature window for producing such glass, making production more complicated.

Features common to the glasses mentioned in the prior art are that they either (1) have a relatively high refractive index n_(d), and/or (2) have low weathering resistance and/or (3) are not X-ray opaque and/or (4) in addition are often difficult or expensive to produce, and/or (5) contain components which are harmful to the environment and/or to health.

SUMMARY OF THE INVENTION

An object of the invention is to provide barium- and lead-free X-ray opaque glasses having a low refractive index n_(d) of about 1.518 to about 1.533. Such glasses should be suitable as dental glass and as optical glass, should be inexpensive to produce, but nevertheless have a high quality and be tolerated by the body, should be suitable for passive and active tooth protection and should have excellent properties with regard to processability, setting behavior of surrounding plastic matrices and long-term stability and strength. In addition, a further object of the invention is that of ensuring that glasses according to the present invention are extremely resistant to weathering.

Such glasses also should have an ALET of at least about 180% and may include (in % by weight based on oxide) SiO₂ 51-<58, B₂O₃>10-<12, Al₂O₃ 3-<7, Li₂O 0-3, Na₂O 0-<3, K₂O 10.5-20, Cs₂O 0-9, ZrO₂>2-8, La₂O₃ 0-15, CaO 0-<0.5, Σ alkali metal oxides 10.5-25, CsO₂+La₂O₃>0.5.

The basic matrix of a glass according to the present invention usually should be free from color-imparting components such as, for example, Fe₂O₃, AgO, CuO etc., in order to permit an optimum color locus and therefore adaptation to the tooth color and, in the case of optical applications, the spectrum of the electromagnetic radiation passing through.

DETAILED DESCRIPTION OF THE INVENTION

Glasses according to the present invention have a refractive index n_(d) of about 1.518 to about 1.533. Such glasses match very well to the available dental plastics and/or epoxy resins in this refractive index range, as a result of which they effectively satisfy the aesthetic demands placed on a dental glass/plastic composite in terms of natural appearance.

Glasses according to the invention achieve the properties of barium- and/or lead-containing dental glasses in terms of the required X-ray absorption without the use of barium and/or lead or other substances harmful to health or classes accordingly. X-ray absorption and therefore X-ray opacity are achieved mainly by the Cs₂O and/or La₂O₃ content; these are present in glasses according to the invention in an amount greater than about 0.5% by weight, either alone or on combination. Both Cs₂O and La₂O₃ are regarded as harmless to health.

Glasses according to the invention also have an ALET of at least about 180%. This means that a small glass plate which is made from glasses according to the invention and has plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 3.6 mm.

In certain embodiments glasses according to the present invention contain SiO₂ in a proportion of about 51 to less than about 58% by weight as a glass-forming component. Higher SiO₂ contents can lead to disadvantageously high melting temperatures, as well as inadequate X-ray opacity.

Certain embodiments of the present invention provide an SiO₂ content of about 52 to less than about 58% by weight. The lower limit of about 52% by weight reduces the tendency for devitrification. Glasses according to the invention also may contain ZrO₂ in a proportion of more than about 2 to at most about 8% by weight. This zirconium content improves to a surprising degree the mechanical properties and, in particular, tensile and compressive strength, and reduces the brittleness of such glasses. In addition, this component promotes X-ray opacity of the glass.

In certain embodiments, ZrO₂ content may be from about 2.1 to about 8% by weight, and in other embodiments from about 2.2 to about 7% by weight.

In addition, it has been discovered that a ratio of SiO₂ and ZrO₂ content of greater than or at least equal to 8 should be maintained, since ZrO₂ is sparingly soluble in silicate glasses and therefore segregation can easily occur. The segregated regions act as centers for scattering light that passes through, analogous to the Tyndall effect. In the case of dental glasses, these centers of scattering impair the aesthetic impression, and therefore segregated glasses are not accepted for dental application; in an optical glass, the centers of scattering generally have an adverse effect on transmission, and so segregated glasses are also undesirable in most optical applications.

B₂O₃ may be present in glasses according to the invention in the range from more than about 10 to less than about 12% by weight. B₂O₃ serves as a flux. Besides reducing the melting temperature, the use of B₂O₃ simultaneously improves the crystallization stability of glasses according to the invention. Contents of higher than about 12% by weight are not recommended in this system in order to avoid impairing good chemical resistance.

Glasses according to the invention also may contain Al₂O₃ in a range from about 3 to less than about 7% by weight. Among other things, Al₂O₃ provides good chemical resistance. However, an Al₂O₃ content of about 7% by weight should not be exceeded in order to avoid increasing the viscosity of the glass, particularly in the hot-processing range, to such an extent that it is difficult to melt the glass. Contents of higher than about 7% by weight are also disadvantageous for the melting of ZrO₂-containing glass. Glasses according to the invention therefore preferably contain from about 3 to about 6% by weight Al₂O₃. In order to make it easier to melt glasses according to the present invention, the sum total of alkali metal oxides present in such glasses may be from at least about 10.5% by weight to at most about 25% by weight. However, alkali metal oxides may reduce the chemical resistance of a glass. The total content of alkali metal oxides in certain embodiments is preferably from about 11 to about 24% by weight and in other embodiments preferably from about 12 to about 23% by weight.

Individually, the content of the alkali metal oxides in glasses according to the present invention may be from about 10.5 to about 20% by weight K₂O, 0 to about 3% by weight Li₂O, 0 to about 3% by weight Na₂O and 0 to about 9% by weight Cs₂O.

K₂O promotes, to a certain extent, improved melting of SiO₂- and ZrO₂-containing glasses. Certain embodiments of the present invention contain about 11 to about 20% by weight K₂O, and other embodiments preferably contain from about 11 to about 19% by weight K₂O.

In certain embodiments, the Li₂O content may be from 0 to about 2% by weight, and in other embodiments, from 0 to about 1% by weight. Glasses according to the certain embodiments of invention are preferably free of Li₂O.

Glasses according to the present invention are also preferably free from CeO₂.

Cs₂O promotes melting properties and simultaneously serves to increase X-ray opacity and to set the refractive index. Glasses according to certain embodiments of the invention may contain from 0 to about 9% by weight Cs₂O, other embodiments preferably contain from 0 to about 8% by weight, further embodiments preferably contain from 0 to about 6% by weight, and still further embodiments preferably contain from 0 to about 5% by weight.

As already described, glasses according to certain embodiments of the present invention should meet the following condition: Cs₂O+La₂O₃>0.5% by weight. This means, for example, that if glasses according to the invention do not contain any Cs₂O, it should contain more than about 0.5% by weight La₂O₃ in order to produce the required X-ray opacity.

Glasses according to the invention may contain from 0 to about 15% by weight La₂O₃ itself. As described, La₂O₃, possibly together with Cs₂O and/or ZrO₂, promotes X-ray opacity of the glass. If no La₂O₃ is present, glasses according to the invention should contain at least about 0.5% by weight Cs₂O.

The La₂O₃ content in certain embodiments is preferably from about 0.5 to about 14% by weight, and in other embodiments preferably from about 2 to about 13% by weight.

CaO may be used to finely set the refractive index and/or X-ray opacity but, when present in larger amounts, may reduce the chemical resistance of the glass. Glasses according to the invention may contain from 0 to about 0.5% by weight CaO. However, the upper limit for CaO in certain embodiments is preferably less than about 0.4% by weight, and in other embodiments is preferably less than about 0.3% by weight.

In order to achieve high X-ray opacity and correspondingly high values of ALET, certain embodiments of the invention provide for the sum total of Cs₂O and/or La₂O₃ present in the glass to be from about 1 to about 21% by weight, in other embodiments preferably from about 2 to about 19% by weight, in further embodiments preferably from about 2 to about 17% by weight, in additional embodiments preferably from about 2 to about 16% by weight, in still further embodiments preferably from about 3 to about 15% by weight and in other embodiments preferably from about 3 to about 14% by weight.

According to certain embodiments of the present invention, further substances may be added to the substances already mentioned. Therefore, it is possible for glasses according to the invention to additionally contain the alkaline-earth metal oxide MgO and/or ZnO in an amount of up to about 2% by weight in each case.

WO₃ and/or Nb₂O₅ and/or HfO₂ and/or Ta₂O₅ and/or Gd₂O₃ and/or Sc₂O₃ and/or Y₂O₃ may additionally be present individually or in any desired combination in an amount of from 0 to about 3% by weight in each case.

In certain embodiments of the present invention from 0 to about 2% by weight of SnO₂ may optionally be present as well.

As already described, glasses according to the invention should be free from BaO and toxic PbO which are classified as harmful to health. The addition of other substances harmful to the environment and/or to health is preferably avoided. In particular, a preferred glass according to the invention also does not contain any SrO because SrO is likewise not accepted in applications relating to health.

Embodiments of the present invention also provide glasses which are preferably free from other components not mentioned in the claims and/or in the present description, i.e. according to such embodiments, the glass consists essentially of the components mentioned. The expression “consists essentially of” here means that other components are present, at most, as impurities, but are not deliberately added to the glass composition as individual components.

However, the invention also provides for the use of glasses according to the invention as a basis for further glasses, in which up to about 5% by weight of further components can be added to glasses according to the invention. In such cases, glasses may consist of at least about 95% by weight of glasses according to the invention.

All of the glasses according to the invention are noted for surprisingly good chemical resistance, which results in a high degree of unreactivity in cooperation with the resin matrix and thus provides a very long service life of dental compositions.

In embodiments of the present invention it is also possible to adapt the color appearance of the glass by adding oxides customary for this purpose. Oxides suitable for imparting color to glasses are known to a person skilled in the art; examples which may be mentioned are CuO and CoO which, for this purpose, can preferably be added in such embodiments in amounts from 0 to about 0.1% by weight.

The present invention also includes glass powders made from glasses according to the invention. Such glass powders may be produced by known processes, such as, for example, those described in DE 41 00 604 C1. Glass powders according to the invention preferably have a mean grain size of up to about 40 μm. In certain embodiments mean grain size of up to about 20 μm are preferred. In other embodiments mean grain sizes from about 0.4 to about 4 μm are preferred. In other embodiments nanopowders having mean grain sizes of 50 to 400 nm are preferred. Other grain sizes and/or grain size distributions are also encompassed by the invention. The above-mentioned glass powder can generally serve as starting material for the use of glasses according to the invention as fillers and/or dental glasses.

In another embodiment of the invention, the surface of the glass powder is silanized using conventional methods. Silanization allows the bonding of the inorganic fillers to the plastic matrix of the plastic dental composition to be improved.

As already described, glasses according to the present invention may be used as dental glass. Such glasses may be employed as fillers in composites for dental restoration, particularly for those based on epoxy resin which require such fillers to be substantially chemically inert. It is also within the scope of the invention for glasses according to the invention to be used as an X-ray opacifier in dental compositions. Such glasses are suitable for replacing expensive crystalline X-ray opacifiers, such as, for example, YbF₃.

Accordingly, the present invention also includes dental glass/plastic composites which contain glasses described herein in the form of dental glass, preferably in the form of glass powders mentioned above. The plastics and other components may be all of the same type and/or mixtures of plastics suitable for dental applications.

In other embodiments of the invention, glasses may be used for producing a dental glass/plastic composite which contains dental plastic, wherein the dental plastic may be a UV-curable resin based on acrylate, methacrylate, 2,2-bis[4-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane (bis-GMA), urethane methacrylate, alkanediol dimethacrylate or cyanoacrylate.

The present invention also includes optical elements which contain glasses according to the present invention. Optical elements are understood to encompass components which can be used for a wide variety of optical applications. These include components through which light passes. Examples of such components include cover glasses and/or lens elements, in addition to carriers of other components, such as, for example, mirrors and glass fibers.

Cover glasses may be used to protect electronic components which include optoelectronic components. Cover glasses are usually present in the form of glass plates having plane-parallel surfaces and are preferably fitted above the electronic component, such that the latter is protected against environmental effects while allowing electromagnetic radiation, such as light, to pass through the cover glass and interact with the electronic component. Examples of such cover glasses include optical caps, elements for the protection of electronic image sensors, cover wafers in wafer level packaging, cover glasses for photovoltaic cells and protective glasses for organic electronic components. Further applications for cover glasses are well known to a person skilled in the art. It is also possible for optical functions to be integrated in the cover glass, for example, when the cover glass is provided at least in regions with optical structures which may preferably be in the form of lenses. Cover glasses provided with microlenses are often used as cover glasses for image sensors of digital cameras, the microlenses usually focusing light impinging obliquely on the image sensor onto the individual sensor elements (pixels).

Since glasses according to embodiments of the present invention are substantially chemically inert, they also may be suitable for applications such as substrate glass in photovoltaics, both for covering silicon-based photovoltaic cells and organic photovoltaic cells and as carrier material of thin-film photovoltaic modules. X-ray absorption of glasses according to the invention have, inter alia, particular advantages when employing photovoltaic modules in space travel, since the latter can be exposed to particularly intense X-radiation outside the Earth's atmosphere.

Glasses according to the present invention are also suitable for use as substrate glass for biochemical applications, in particular for molecular screening processes.

In other embodiments the high thermal stability of glasses according to the invention allows them to be suitable as lamp glass, in particular for use in halogen lamps. If the light generation mechanisms in the lamp produce X-radiation, a particular advantage of glasses according to the invention is that they can keep X-radiation away from the surroundings.

In addition, embodiments of the invention include the evaporation of glasses described herein by means of physical processes and the deposition of evaporated glass on certain components. Such physical vapor deposition processes (PVD processes) are known to a person skilled in the art and are described, for example, in DE 102 22 964 B4. Here, glasses according to the present invention serve as targets to be evaporated in such processes. Components which are evaporation-coated with glasses according to the invention can benefit both from the chemical resistance of the glass and from X-ray absorption thereof.

It is also possible for embodiments of the present invention to be used as starting material for glass fibers. The term “glass fiber” encompasses all types of glass fibers, in particular fibers comprising only a core, and so-called core-shell fibers having a core and at least one shell which preferably completely surrounds the core along the outer circumferential surface. Glasses according to the present invention may be used as core glass and/or as shell glass. Within the composition range of glasses according to the invention, the refractive index n_(d) of the glass can be set such that a core glass according to the invention has a higher refractive index than a shell glass according to the invention, forming a so-called step-index fiber in which light is conducted very efficiently by total reflection at the core-shell interface.

Because of its good chemical resistance, glass fibers according to the present invention may be used as reinforcements in composite materials and/or as reinforcements for concrete and/or as optical fibers embedded in concrete.

Examples 1-11

Table 1 below provides 11 representative embodiments of the present invention labeled Example 1 through Example 11. All quantitative descriptions of the particular components of these embodiments are given in % by weight (based on oxide).

Glasses described in these Examples were produced as follows:

The raw materials for the oxides were weighed out without refining agents and then thoroughly mixed. The glass batch was melted down at about 1550° C. in a batchwise melting unit, then refined and homogenized. The glass may be poured at a temperature of about 1550° C. as ribbons or with other desired dimensions, and processed. The temperatures may be reduced by at least about 100 K in a large-volume, continuous unit.

For further processing, the cooled glass ribbons were milled with the aid of the process described in DE 41 00 604 C1, the contents of which are incorporated by reference herein, to form a glass powder with a mean grain size of about 10 μm or less. The glass properties were determined on the basis of glass gobs which had not been milled into powders. All of these glasses have excellent chemical resistance with respect to acids, alkalis and water. In addition, they are very chemically inert.

Table 1 also lists the refractive indexes n_(d), the coefficients of linear thermal expansion α_((20-300° C.)) from 20 to 300° C. and α(−30-70° C.) from −30 to 70° C. The latter is of particular interest when glasses according to the invention are used as dental glass because the temperature range from −30 to 70° C. can occur during use.

Table 1 also lists the aluminum equivalent thickness (ALET) and the chemical resistance of each of the Examples provided therein. In Table 1 SR represents the acid resistance class according to ISO8424, AR represents the alkali resistance class according to ISO10629 and HGB represents the hydrolytic resistance class according to DIN ISO719.

All of the glasses listed in Table 1 have coefficients of thermal expansion a in the range from 20 to 300° C. of less than 8·10⁻⁷/K.

Glasses shown in Table 1 have an X-ray opacity which is at least as good as that of glasses containing BaO and SrO. In the Examples shown, ALET values of 255% to 472% were obtained. In addition to Example 10, Example 11 shows one of the highest X-ray absorption values and the highest ALET value. This Example contains not only Cs₂O and La₂O₃ but also SnO₂ which contributes to the X-ray opacity. By contrast, Example 11 contains a maximum total of 8.41% by weight Cs₂O and 4.38% by weight La₂O₃. Accordingly, the refractive index n_(d) of Example 10 is among the highest, at 1.53028.

It is also of interest to compare Example 6 with Example 3 which, with a refractive index n_(d)=1.52861, has only a slightly lower refractive index than Example 6 but, with an ALET value of 270%, has an X-ray absorption that is still good but significantly lower than that of Example 6. Example 3 contains significantly less La₂O₃ than Example 6, and therefore more ZrO₂. This suggests that La₂O₃ may contribute to stronger X-ray absorption than ZrO₂.

Example 5 does not contain any La₂O₃, but contains 4.09% by weight Cs₂O. The refractive index of Example 5, at n_(d)=1.52224, is less than that of Example 3, and its ALET is 276%. This demonstrates good X-ray absorption properties which can be achieved by the addition of Cs₂O in the case of certain embodiments of the present invention.

A feature common to all of these Examples listed in Table 1 is that their chemical resistance can be classed in the best SR, AR and HGB classes 1 or 1.0, such that these glasses are therefore very suitable for the uses mentioned.

The Examples also demonstrate that the refractive indexes n_(d) of glasses according to the invention may be adapted to the intended application within an appropriate range around 1.525, without adversely affecting the outstanding chemical resistance. As a result, such glasses may be advantageously used in particular as fillers in dental compositions and for other applications which impose high demands, inter alia, on purity, chemical resistance and thermal stability. Such glasses also may be produced on a large industrial scale at a reasonable cost.

Compared to the prior art, glasses according to the invention have the further advantage that they link the adaptability of the refractive indexes and coefficients of expansion and provide surprisingly good chemical stability with efficient X-ray absorption.

In addition, glasses according to the present invention are surprisingly easy to melt and therefore can be produced at a reasonable cost.

TABLE 1 Compositions of X-ray opaque glass in % by weight Example 1 2 3 4 5 6 7 8 9 10 11 SiO₂ 57.77 57.05 55.90 55.13 56.26 52.96 56.08 56.03 53.25 52.69 52.09 B₂O₃ 11.16 11.08 11.05 10.90 11.13 10.47 11.09 11.16 10.53 10.42 10.30 Al₂O₃ 5.41 5.38 5.36 5.29 5.40 5.08 5.38 5.42 5.11 5.06 5.00 Li₂O 0.45 Na₂O 2.7 2.69 2.68 2.70 2.54 2.69 2.63 2.55 2.53 0.65 K₂O 11.82 12.25 13.93 17.75 14.02 13.20 13.97 13.31 13.27 13.13 12.98 Cs₂O 4.09 2.04 4.04 3.49 5.31 8.41 CaO 0.1 La₂O₃ 4.6 5.16 4.69 4.63 12.65 2.35 0.53 8.68 8.59 4.38 ZrO₂ 6.44 6.39 6.38 6.29 6.42 3.10 6.4 6.44 3.12 2.27 4.82 SnO₂ 1.38 n_(d) 1.52470 1.52643 1.52861 1.52540 1.52224 1.52862 1.52510 1.52526 1.53128 1.53028 1.52553 α_((20-300° C.)) 7.02 7.26 7.64 7.40 7.58 7.65 7.57 7.64 7.99 8.19 7.03 [10⁻⁷/K] α_((−30-70° C.)) 6.78 [10⁻⁷/K] ALET [%] 255 277 270 274 276 359 274 284 371 395 472 SR [class] 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 AR [class] 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 HGB [class] 1 1 1 1 1 1 1 1 1 1 1 

1. A BaO- and/or PbO-free X-ray opaque glass having a refractive index n_(d) of about 1.518 to about 1.533 and an aluminum equivalent thickness of at least about 180%, comprising (in % by weight based on oxide) SiO₂   51-<58 B₂O₃ >10-<12 Al₂O₃   3-<7 Li₂O 0-3 Na₂O   0-<3 K₂O 10.5-20   Cs₂O 0-9 ZrO₂ >2-8   La₂O₃  0-15 CaO     0-<0.5 Σ alkali metal oxides 10.5-25   CsO₂ + La₂O₃ >0.5.


2. The glass of claim 1, comprising SiO₂   52-<58 B₂O₃ >10-<12 Al₂O₃ 3-6 Li₂O 0-2 Na₂O 0-3 K₂O 11-20 Cs₂O 0-8 ZrO₂ 2.1-8   La₂O₃  2-14 CaO     0-<0.4 Σ alkali metal oxides 11-24 CsO₂ + La₂O₃   2-22.


3. The glass of claim 1, comprising SiO₂   52-<58 B₂O₃ >10-<12 Al₂O₃ 3-6 Li₂O 0-1 Na₂O 0-3 K₂O 11-19 Cs₂O 0-6 ZrO₂ 2.2-7   La₂O₃  2-13 CaO     0-<0.3 Σ alkali metal oxides 12-23 Cs₂O + La₂O₃   2-19.


4. The glass of claim 1, wherein the sum total of the Cs₂O and/or La₂O₃ content is between about 2% and about 17%.
 5. The glass of claim 1, wherein the ratio of the SiO₂ and ZrO₂ contents is: SiO₂/ZrO₂≧8.
 6. The glass of claim 1, further comprising ZnO₂ 0-2 MgO 0-2 WO₃ 0-3 Nb₂O₅ 0-3 HfO₂ 0-3 Ta₂O₅ 0-3 Gd₂O₃ 0-3 Sc₂O₃ 0-3 Y₂O₃ 0-3 SnO₂  0-2.


7. The glass of claim 1, wherein said glass is free of SrO.
 8. A glass comprising at least 95% (in % by weight based on oxide) of the glass of claim
 1. 9. A glass powder comprising the glass of claim
 1. 10. The glass powder of claim 9, wherein the surfaces of powder grains are silanized.
 11. A dental glass/plastic composite comprising the glass powder of claim
 9. 12. An optical element comprising the glass of claim
 1. 13. A dental glass comprising the glass of claim
 1. 14. A filler in composites for dental restoration comprising the glass of claim
 1. 15. A dental glass/plastic composite comprising the glass of claim 1, wherein said dental plastic comprises a UV-curable resin based on acrylate, methacrylate, 2,2-bis[4-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane (bis-GMA), urethane methacrylate, alkanediol dimethacrylate or cyanoacrylate.
 16. An X-ray opacifier in plastic dental compositions comprising the glass of claim
 1. 17. An optical device comprising the glass of claim
 1. 18. A cover glass for an electronic component comprising the glass of claim
 1. 19. The cover glass of claim 18, wherein the electronic component is a sensor.
 20. A display device comprising the glass of claim
 1. 21. A substrate glass in a photovoltaic device comprising the glass of claim
 1. 22. A substrate glass and/or cover glass in an OLED device comprising the glass of claim
 1. 23. A lamp glass comprising the glass of claim
 1. 24. A substrate glass for biomedical devices comprising the glass of claim
 1. 25. A target material for PVD processes comprising the glass of claim
 1. 26. A glass fiber comprising core glass and/or shell glass wherein said core glass and/or shell glass comprises the glass of claim
 1. 